Scalar transport after a high-resolution solitary fractal tree based on large-eddy simulation: Implication to urban green infrastructure

Scalar transport is one of the most concerned issues in the urban environment (e.g., pollutants, pollens, and aeolian transport). Vegetations are commonly regarded as scalar-moderation structures in urban areas by influencing airflows, avoiding aeolian erosion and pollution. The morphological multi-scale branch structures of the single tree were usually neglected in previous studies, which, however, can lead to highly inhomogeneous wake. The turbulence generation through the multi-scale branches is not yet clear. The corresponding spatial details of scalar distribution and its mechanism need to be further examined. In this study, the wake after a single fractal tree is investigated using the large-eddy simulation (LES). The spatial characteristics of the wakes are illustrated, indicating susceptible zones in the canopy, first-generation levels of branches, and recirculation regions. Higher-order moments signify the rare, extreme events with strong upward acceleration, removing air masses likely to occur at the canopy shear layer within 3h downstream where h is the characteristic tree height. The quadrant analysis elucidates the scalar transport potential. The scalar distribution and its transport mechanism are visualized, showing dilution and accumulation by the streamwise and vertical velocities. Involving the multi-scale geometries of the tree can provide guidance for urban green infrastructure (UGI) planning, advancing environmental and sustainability development. •Flows and scalar transport around a solitary fractal tree are calculated by LES.•Reverse flows consist of updraft/downdraft where the scalar concentration is uniform.•Higher-order-moment analysis illustrates the extreme events and the flow interaction.•Trees downstream within 3h could be affected by the shear from the trees upstream.•Scalar concentration fluctuates more intensely in the reversed flows and shear layers.